Obligate phototrophy in cyanobacteria: more than a lack of sugar transport.

DNA hybridization using the Synechocystis PCC6803 glucose transporter gene, glcP, revealed a single homologous region in two facultative photoautotrophic strains out of three tested, and none in three obligate autotrophs. In one of the latter, Synechococcus PCC7942, integration of glcP into the chromosome resulted in glucose sensitivity. A subclone isolated as glucose-tolerant had lost glcP. Integration in a replicative vector allowed glucose transport and photoheterotrophic growth, but could not be maintained. Thus lack of sugar transport could explain cyanobacterial obligate autotrophy. However, at least in Synechococcus PCC7942, acquisition of such a transport capacity created a metabolic disequilibrium barely compatible with survival.